Many modern electronic devices require flame retardant approval from Underwriters Laboratories (UL). These include such devices as personal and business computers, various radio frequency and microwave devices, equipment used in telephone base stations and switching electronics. If each individual component of such apparatus has UL approval, the overall apparatus does not require flame-retardant approval. Thus, ensuring that each component has UL approval avoids the need for UL testing of the entire apparatus and reduces cost to the apparatus manufacturer.
The need for flame retardant approval of individual components extends to fabric materials that may be used in various shielding components of the apparatus. Shielding components protect the electrical or electronic components of the apparatus from electromagnetic interference (EMI). Electromagnetic interference is understood to mean undesired conducted or radiated electrical disturbances from an electric or electronic apparatus, including transients, which can interfere with the operation of other electrical or electronic apparatus. Such disturbances can occur anywhere in the electromagnetic spectrum. Radio frequency interference (RFI) refers to disturbances in the radio frequency portion of the electromagnetic spectrum but often is used interchangeably with electromagnetic interference. Both electromagnetic and radio frequency interference are referred to hereafter as EMI.
Electronic devices not only are sources of EMI, but also the operation of such devices may be adversely affected by the emission of EMI from other sources. Consequently, electric or electronic apparatus susceptible to electromagnetic interference generally must be shielded in order to operate properly.
Many shielding applications such as gaskets, cable shields, grounding straps, conductive tapes, laminate shields among others, utilize a conductive fabric in its construction. For example, a gasket for use between a computer cabinet and a cabinet door may comprise a resilient core enclosed in a conductive fabric. Conductive fabrics generally are formed of polymeric fibers and are either woven or non-woven. To render the fabric conductive, the fibers may include particles of a conductive material or the fabric may be coated with a conductive metal by various methods including electroless plating and vapor deposition among others.
One method of providing a conductive fabric with flame retardant properties is to incorporate a flame retardant into the material of the fabric. For example, U.S. Pat. No. 5,674,606 discloses dispersing alumina trihydrate in a polymeric material used to form a conductive fabric. A further alternative is to form the fabric of fiberglass. While a fiberglass fabric is inherently fire resistant, it is brittle and subject to cracking in dynamic applications. Substrate fabrics of polymeric materials generally are more flexible and durable than fiberglass and are preferred. The problem is that prior attempts to produce a conductive polymeric fabric having flame-retardant properties suitable for use as an EMI shield have not been entirely satisfactory.
The industry standard for a flame retardant EMI shielding fabric is a fabric having an Underwriters Laboratories rating for very thin material (VTM) of zero burn in a vertical burn test (described hereinbelow). A VTM burn rating of zero is particularly difficult to achieve for metalized polymeric fabrics because the metal coating acts as an accelerant to combustion.
Incorporating a flame retarding material into the formulation of the polymeric material of the fabric provides a degree of protection but does not completely solve the problem. Applying a flame-retardant material over the conductive metalized surface may provide a UL approved material. However, the amount of flame retardant that must be applied over the metalized surface in order to obtain the UL VTM zero burn rating (vertical burn test) forms such a thick layer that it significantly decreases the surface conductivity of the metalized fabric. Since high surface conductivity is a desirable attribute of EMI shielding material, a material having a low surface conductivity renders it unacceptable for such use. Low surface conductivity also is caused by corrosion of the conductive metal layer and conventional flame-retardant materials accelerate galvanic corrosion of the conductive metal. This is another reason why applying a flame-retardant coating to the metalized surface of a conductive fabric has not been an acceptable solution.
Accordingly, it is an object of the present invention to provide a electrically conductive polymeric fabric having flame retardant properties.
Another object of the present invention is to provide a conductive polymeric fabric that has an Underwriters Laboratories vertical burn test VTM flammability rating of zero.
A further object of the present invention is to provide a flame retardant conductive polymeric fabric that is corrosion resistant so as to maintain a high degree of surface conductivity over time.
Yet another object of the present invention is to provide method of making a flame retardant conductive polymeric fabric suitable for use in EMI applications.